Return the angle between two points, which is also equal to the distance
between these points on the unit sphere. The points do not need to be
normalized. This function has a maximum error of 3.25 * DBL_EPSILON (or
2.5 * double.epsilon for angles up to 1 radian).
Return the angle between two points, which is also equal to the distance between these points on the unit sphere. The points do not need to be normalized. This function has a maximum error of 3.25 * DBL_EPSILON (or 2.5 * double.epsilon for angles up to 1 radian).